Preparation And Properties Of Manganese Based Complexes As Model For Photocatalytic Oxidation Of Water Oxygen Release

Water oxidation（2H₂O→4H⁺+4e^-+O₂）is a key step in utilization of solar energy for producing hydrogen of artificial photosynthetic system.Inspired by the oxygen evolution complex（OEC）in PSII,molecular catalysts are extensively studied and the progress both in biomimetic studies and the mechanistic understanding is truly remarkable.While research efforts in the earlier years focused on the Ru-,Mn-and Ir-based complexes,the focus has gradually changed to non-precious catalysts based on earthabundant metals（Fe,Co,Ni,Cu）.These studies have deepened our understanding on the mechanism of water oxidation,including both proton-coupled electron transfer and O-O bond formation.Photosynthetic splitting of water into oxygen by plants,algae,and cyanobacteria is catalyzed by the oxygen-evolving center（OEC）.The active site of OEC is an asymmetric[Mn₄Ca O₅]pentagonal heteronuclear metal cluster,which consists of a cubane structure[Mn₃Ca O₄]core with aμ₄-O,aμ₂-OH and two carboxyl groups attached to a Mn ion outside the cubane.Its synthetic simulation may facilitate the study of the structural and chemical determinants of biological water oxidation and advance the development of advanced catalysts for artificial photosynthesis.The main purpose of this paper is to synthesize manganese complex which can be used as biomimetic model compounds of OEC by means of coordination chemistry,and explore the properties of manganese complex catalyzing water oxidation.The first chapter is the introduction,which briefly introduces the construction of OEC-like catalytic system and the research progress of bioexcited multi-nuclear catalysts for water oxidation and the significance of the topic.In the second chapter of this paper,the trisubstituted methanol ligand HL₁:tris（2-pyridyl）methanol and H₄L₂:tris（2-phenol）methanol are successfully synthesized inmethanol,and six metal complexes are methanol through different coordination experiments:[Mn₄^II（L₁）₄]·（Cl O₄）₄（1）[Mn₄^II（L₁）₄]·[Mn^II（NO₃）₄]·（NO₃）₂（2）[Mn₄^II（L₁）₄]·[Ce₂^Ⅲ（OAc）₁₀]（3）[Mn₄^II（L₁）₃（NO₃）₃（OH）]·[Mn_0.5^II（NO₃）₂]（4）[Mn^ⅢMn^II ₃（L₁）₃（L₂）]·[Mn^II（NO₃）₄]（5）The six metal complexes all have"cubane"structure,but their crystal space groups are different.The differences of coordination environment and counterbalance ions lead to different crystal structures.Interestingly,[Mn₄^II（L₁）₄]·[Mn^II（NO₃）₄]·（NO₃）₂（2）can be changed to[Mn₄^II（L₁）₃（NO₃）₃（OH）]·[Mn_0.5^II（NO₃）₂]（4）and[Mn^ⅢMn₃^II（L₁）₃（L₂）]·[Mn^II（NO₃）₄]（5）through ligand exchange in different ligand environment.Their crystal structure and elemental analysis were characterized,and their magnetic properties were studied.In the third chapter of this paper,the catalytic capacity of some mangan-based complexes for water oxidation was investigated.Respectively for[Mn₄^II（L₁）₄]·（Cl O₄）₄（1）and[Mn₄^II（L₁）₃（NO₃）₃（OH）]·[Mn_0.5^II（NO₃）₂]（4）for the electrochemical test.According to the CV and DPV test results,[Mn₄^II（L₁）₄]·（Cl O₄）₄（1）and[Mn₄^II（L₁）₃（NO₃）₃（OH）]·[Mn_0.5^II（NO₃）₂]（4）can catalyze the oxidation of water because there is a significant catalytic current at a potential of 1.3V（vs Fc/Fc⁺）.However,when the voltage of 1.3V（vs Fc/Fc⁺）is applied for potentiostatic electrolysis,the catalytic current is unstable and increases over time.And the formation of obvious brown oxides can be observed on the ITO electrode.This indicates that[Mn₄^II（L₁）₄]·（Cl O₄）₄（1）and[Mn₄^II（L₁）₃（NO₃）₃（OH）]·[Mn_0.5^II（NO₃）₂]（4）catalyzed water oxidation at the constant potential of 1.3V（vs Fc/Fc⁺）are not homogeneous catalysis.However,this result provides support for the subsequent use of photocatalysis to oxidize water.